This invention relates to the general field of combustion systems and more particularly to a multi-stage, multi-plane, low emissions combustion system for a small gas turbine engine.
In a small gas turbine engine, inlet air is continuously compressed, mixed with fuel in an inflammable proportion, and then contacted with an ignition source to ignite the mixture which will then continue to burn. The heat energy thus released then flows in the combustion gases to a turbine where it is converted to rotary energy for driving equipment such as an electrical generator. The combustion gases are then exhausted to atmosphere after giving up some of their remaining heat to the incoming air provided from the compressor.
Quantities of air greatly in excess of stoichiometric amounts are normally compressed and utilized to keep the combustor liner cool and dilute the combustor exhaust gases so as to avoid damage to the turbine nozzle and blades. Generally, primary sections of the combustor are operated near stoichiometric conditions which produce combustor gas temperatures up to approximately four thousand (4,000) degrees Fahrenheit. Further along the combustor, secondary air is admitted which raises the air-fuel ratio (AFR) and lowers the gas temperatures so that the gases exiting the combustor are in the range of two thousand (2,000) degrees Fahrenheit.
It is well established that NOx formation is thermodynamically favored at high temperatures. Since the NOx formation reaction is so highly temperature dependent, decreasing the peak combustion temperature can provide an effective means of reducing NOx emissions from gas turbine engines as can limiting the residence time of the combustion products in the combustion zone. Operating the combustion process in a very lean condition (i.e., high excess air) is one of the simplest ways of achieving lower temperatures and hence lower NOx emissions. Very lean ignition and combustion, however, inevitably result in incomplete combustion and the attendant emissions which result therefrom. In addition, combustion processes are difficult to sustain at these extremely lean operating conditions. Further, it is difficult in a small gas turbine engine to achieve low emissions over the entire operating range of the turbine.
Significant improvements in low emissions combustion systems have been achieved, for example, as described in U.S. Pat. No. 5,850,732 issued Dec. 22, 1998 and entitled xe2x80x9cLow Emissions Combustion Systemxe2x80x9d assigned to the same assignee as this application and incorporated herein by reference. With even greater combustor loading and the need to keep emissions low over the entire operating range of the combustor system, the inherent limitations of a single-stage, single-plane, combustion system become more evident.
The low emissions combustion system of the present invention includes a generally annular combustor formed from a cylindrical outer liner and a tapered inner liner together with a combustor dome. A plurality of tangential fuel injectors introduces a fuel/air mixture at the combustor dome end of the annular combustion chamber in two spaced injector planes. Each of the injector planes includes multiple injectors delivering premixed fuel and air into the annular combustor. A generally skirt-shaped flow control baffle extends from the tapered inner liner into the annular combustion chamber. A plurality of air dilution holes in the tapered inner liner underneath the flow control baffle introduce dilution air into the annular combustion chamber. In addition, a plurality of air dilution holes in the cylindrical outer liner introduces more dilution air downstream from the flow control baffle.
The fuel injectors extend through the recuperator housing and into the combustor through an angled tube which extends between the outer recuperator wall and the inner recuperator wall and then through the cylindrical outer liner of the combustor housing into the interior of the annular combustion chamber. The fuel injectors generally comprise an elongated injector tube with the outer end including a coupler having at least one fuel inlet tube. Compressed combustion air is provided to the interior of the elongated injector tube from openings therein which receive compressed air from the angled tube around the fuel injector which is open to the space between the recuperator housing and the combustor.
The present invention allows low emissions and stable performance to be achieved over the entire operating range of the gas turbine engine. This has previously only been obtainable in large, extremely complicated, combustion systems. This system is significantly less complicated than other systems currently in use.